NITL converts SocketException to HostUnreachable, apparently due to legacy sharding SDAM logic according to the comment there:

// The TransportLayer has, for historical reasons returned SocketException for network
// errors, but sharding assumes HostUnreachable on network errors.

However, this is no longer true. Remote command targeting / SDAM error handling uses error categories such as NetworkError (which HostUnreachable belongs to) to make these changes:

• https://github.com/mongodb/mongo/blob/e204c648682f23e15c2a44218615fc2074f04fd2/src/mongo/client/remote_command_targeter.h#L89
• https://github.com/mongodb/mongo/blob/e204c648682f23e15c2a44218615fc2074f04fd2/src/mongo/client/streamable_replica_set_monitor_error_handler.cpp#L87-L120

We should remove this conversion logic as it is no longer needed and can obfuscate the intended error codes returned from the TransportLayer.